Exposure to Amorphous Silica Fibers and Other Particulate Matter During Rice Farming Operations

ABSTRACT

Occupational exposure to biogenic amorphous silica fibers was found during all phases of rice farming. Exposure during field preparation was the highest, followed by harvest and then rice stubble burning. The highest personal exposure was 1.9 fibers/cc for fibers >5 μm in length in the respirable dust fraction. The highest level seen in area samples was 9.9 fibers/cc for fibers >5 μm in length in the respirable dust fraction. The median fiber length was 2.8 μm, with a range from 0.5 to 20 μm. (Fibers <0.5 μm were not counted.) Ninety percent of fibers were <9 μm in length. The median fiber width was 0.9 μm, with a range from 0.2 to 7 μm. Ninety percent of fibers were <2.5 μm in width. Samples for airborne amorphous silica fibers were collected with personal sampling pumps and polycarbonate filters. Samples were analyzed using X-ray fluorescence to identify composition and electron diffraction to determine the crystalline state of fibers. Fiber counting methodology was adapted from standard asbestos analytical procedures. The biogenic amorphous silica fibers were of complex morphology, often having no parallel sides. Although the fibers did not have needlelike or hairlike shapes, ends of some fibers were sharply pointed.

MATERIALS AND METHODS

Sample Collection

Samples for airborne amorphous silica fibers were collected with personal sampling pumps (MSA model G) using a 10-mm nylon respirable dust cyclone at a flow rate of 1.7 L/min. Flow rates were periodically checked with a precision rotameter. The sampling medium was a 0.4-μm pore size, 25-mm diameter track-etched polycarbonate membrane filter (Poretics) in front of a 5-μm pore size cellulosic diffuser loaded in a carbon-filled polypropylene cassette with a 50-mm extended cowl. The bracket on the cyclone holder was modified to fit the extended cowl cassette. Some area samples in upwind locations and all downwind and community locations were collected with the same sampling media open face using a high flow pump at a flow rate of 8 to 10 L/min, without use of a cyclone. Ranging studies were performed to determine. optimal loading for each phase of sample collection.

Respirable dust and crystalline silica samples were collected at a flow rate of 1.7 L/min using a respirable dust cyclone and a 5-μm pore size, 37-mm diameter polyvinylchloride (PVC) filter in a polystyrene cassette. Total dust samples were collected at a flow rate of 2.0 L/min with 5-μm pore size, 37-mm diameter PVC filters in polystyrene cassettes. Personal exposure monitoring for carbon monoxide was conducted using diffusion indicator tubes.

Samples were collected during harvest, burning of rice straw after harvest, and field preparation. Field preparation samples were collected in fields that had been burned from 1 day to 2 months previously. Breathing zone samples were collected simultaneously with samples upwind and downwind of the farming operation. When closed cab equipment was used by the farmer, samples were collected both inside and outside the cab. Samples were also collected from outdoor locations in communities in the rice-growing region on prescribed burn days.

When monitoring rice farming operations, one sampler was placed upwind of the operation at a height of approximately 2 m. A minimum of one downwind sampler was used, at a height of approximately 2 m. During burn operations multiple downwind samplers were used. Three samplers were placed at the downwind edge of the field, two at a height of approximately 2 m and a third on a mast approximately 6 m in the air. One sampler was placed approximately 1.5 km downwind at a height of approximately 2 m. Some of these samples were collected open face without use of a cyclone.

Ten percent of collected samples were field blanks. Ten percent of collected samples were side-by-side duplicates. Duplicate samples were taken within 4 cm of each other and were run simultaneously. One set of eight replicates (side-by-side samples) was collected during harvesting and a second set when burning was done with a torch towed by a tractor. A portable weather station was located on site to indicate wind speed, direction, air temperature, relative humidity, and solar radiation during rice harvest and burn operations.

Sample Analysis

The analytical method is discussed in detail by Scales et al. and is only briefly summarized here. Samples were analyzed for amorphous silica fibers using a Hitachi H-600/H-601A transmission electron microscope with electron diffraction and scanning transmission modes, operated at 75 kV. Particles having aspect ratios of three or greater were classified as fibers. Fibers >5 μm in length were tabulated separately. Fibers containing silica were identified by their characteristic fluorescent X rays using a Quantex (thin window) X-ray detector and a Kevex Delta Class Analyzer. Electron diffraction was used to determine the crystalline state of fibers. Fiber counting methodology was adopted from standard asbestos analytical procedures. Reference samples were prepared from ash collected from burns in a wind tunnel at the University of California, Davis, as described by Scales et al. Particles where reported as amorphous silica fibers if they met the fiber definition, contained silica but not aluminum, and showed no Bragg reflections.

Gravimetric analysis was used for respirable dust and total dust samples. Filters were preweighed and postweighed on an electrobalance located in a temperature- and humidity-controlled room. Crystalline silica content was analyzed following National Institute for Occupational Safety and Health (NIOSH) Method 7500 using a Diano XRD 8000 diffractometer, with a copper tube operated at 50 kV and 15 mA. The sample PVC filters were ashed in a muffle furnace, dispersed ultrasonically, and filtered on silver membranes. Standards on silver membranes were prepared from NIOSH reference quartz (Q-1).

The initial intention of the project was to analyze all samples that were collected. The first samples collected were harvest samples, and therefore the first samples analyzed were harvest samples. It soon became clear that not all samples could be analyzed within the time and budget of the project. The laboratory was instructed to analyze samples with the following priority: burn samples, then field preparation samples, then harvest samples. Within that priority guideline, the laboratory was instructed to randomly select the samples to analyze. The laboratory was blinded to the identity of duplicate samples.
RESULTS

Silica Fiber Analysis

The method could distinguish between amorphous and crystalline particles. Amorphous silica fibers were seen on many of the samples. The method could also distinguish whether particles containing silicon also contained aluminum. Fibers that contained silicon but no aluminum were always amorphous. The fibers were of complex morphology, often with no parallel sides. Although the fibers did not have needlelike or hairlike shapes, ends of fibers were sometimes sharply pointed. The morphology of fibers seen on the samples was similar to fibers in the rice ash reference samples. The median fiber length was 2.8 μm, with a range from 0.5 to 20 μm. (Fibers <0.5 μm were not counted.) Ninety percent of fibers were <9 μm in length. The median fiber width was 0.9 μm, with a range from 0.2 to 7 μm. Ninety percent of fibers were <2.5 μm in width. Photographs of amorphous silica fibers are included as Figures 1 through 5.

Approximately 70 percent of all samples were analyzed because of time and budget constraints. The fiber analysis required an average of a full day of electron microscopy per sample. Samples to analyze were randomly selected within the sampling categories of harvest, burn, and field preparation, and the laboratory was blinded to the identity of duplicates. A decision was made to analyze the first six of ten collected blanks. Samples analyzed included 100 percent of burn samples, 75 percent of upwind samples, 70 percent of field preparation  samples, 60 percent of duplicate samples, and 50 percent of harvest samples. Approximately 10 percent of all samples analyzed were too overloaded to yield fiber count results. These were primarily samples which had been run longer than 15 minutes. Five pairs of duplicates that were analyzed had coefficients of variation ranging from 4 to 33 percent, with an average of 18 percent for the numbers of fibers detected per sample.

To prevent overloading, sampling times were limited to about 15 minutes for most filters. This gave an effective detection limit of 0.1 f/cc for these samples. Sampling times for downwind, town, and closed cab interior samples were longer and had effective detection limits ranging from 0.02 to 0.004 f/cc.

Exposure to Amorphous Silica Fibers

A total of 86 samples were analyzed. These samples represented 52 distinct personal exposures or area sampling locations, as several samples were sometimes taken for one personal or area exposure location. All results are for fibers >5 μm in length and represent the calculated time-weighted average level for the personal or area exposure location.

Eleven samples collected upwind of rice farming operations were used to control for ambient background levels of amorphous silica fibers. Amorphous silica fibers were observed on 1 of the 11 samples, giving a result of 0.02 f/cc. The mean for all 11 samples was less than 0.02 f/cc (below the limit of detection for those samples). Field blanks were used to control for contamination of the sampling media. No amorphous silica fibers were detected on six blanks.

Amorphous silica fibers were found on most personal and area samples collected at locations within rice fields. In 14 personal exposures, airborne time-weighted average levels for amorphous silica fibers ranged from none detected to 1.9 f/cc (Table 1). Personal exposures were higher for operators of open cab equipment than for operators of closed cab equipment. Personal exposures were lowest for employees on foot and burning rice stubble. Mean levels for burning on foot were <0.1 f/cc (below the limit of detection for those samples). Mean levels for open cab samples were: 1.8 f/cc, fire-lighting from a tractor; 1.1 f/cc, bulldozer used for field preparation; 0.3 f/cc, bank out wagon used for rice harvest. Mean levels for closed cab samples were: 1.02 f/cc, tractor used for field preparation; 0.13 f/cc, harvester.

Fourteen area samples were collected on the exterior of closed cab harvesters and tractors performing field preparation, representing a total of six different area exposures. These samples ranged from 0.4 to 9.9 f/cc. Mean levels on tractors performing field preparation were 8.4 f/cc. Mean levels on harvesters were 1.2 f/cc.

We compared the difference between measurements taken on the same day on the interior and exterior of air-conditioned closed cab equipment performing the same operation to estimate the level of protection provided by the enclosed cab. Personal exposures inside the cab ranged from 2 to 19 percent of levels measured on the cab exterior. It was clear that some of the exposure in the cab interior was due to entry of outside air when the equipment operator entered and exited the cab periodically to adjust settings or clear jams. Also, a window in the cab was sometimes left open during the nicest part of the day.

Amorphous silica fibers were observed on one of two 1.5-km downwind samples and on two of four field edge downwind samples. The mean level was 0.004 f/cc for all downwind samples. The mean level for the three samples on which fibers were seen was 0.014 f/cc. For community samples, collected on days when there was rice burning, fibers were seen on 4 of the 14 samples. The mean level for all town samples was <0.004 f/cc (the limit of detection for those samples). The range for the four samples on which fibers were seen was 0.005 to 0.010 f/cc (Table 1). As some of these samples were collected without use of a cyclone, these results may be biased to the high side.

Using the SAS general linear models procedure to analyze variance on the log transformation of the data, airborne fiber levels could be divided into four groups, from highest to lowest: (1) field preparation; (2) burning using a torch towed by a tractor and harvest; (3) exposures inside closed cab equipment; (4) burning by employees on foot, upwind, downwind, and town (p < 0.0001 for the difference between each group) (Table 2).

Exposure to Other Airborne Contaminants

Measurements were made of airborne levels of total dust, respirable dust, and crystalline silica. One personal exposure sample from an open cab tractor towing a torch gave results of 1.83 mg/m3 total dust, 0.30 mg/m3 respirable dust, and 0.07 mg/m3 respirable quartz. Two exposure samples for personnel burning on foot gave respirable dust levels of 0.14 and 0.31 mg/m3 and respirable quartz levels of 0.02 and 0.03 mg/m3. Cristobalite and trydimite were not detected on any of the samples (Table 3).

Area measurements on the exterior of closed cab equipment were collected for total dust, respirable dust, and respirable quartz measurements on 5 days of harvest and 3 days of field preparation. Levels ranged from 4.02 to 72.1 mg/m3 for total dust, 0.52 to 5.24 for respirable dust, and 0.02 to 0.09 for respirable quartz. Field preparation resulted in higher levels than harvest. Cristobalite and trydimite were not detected on any of the samples (Table 3).

Carbon monoxide levels were measured on 4 days of burning. Five personal samples were taken, as well as one sample at the field edge downwind of burning. Levels did not reach the limit of detection of the diffusion indicator tubes (which ranged from 25 to 50 ppm carbon monoxide).
